Antilock Braking Systems (ABS) were originally invented for the aviation industry to prevent wheel skid during landings. Early ABS systems were hydraulic. Problems with wheel speed systems, such as inability to operate reliably on rough terrain, during cornering, when fouled with contaminants, or when exposed to high heat prevented ABSs from being implemented in automobiles.
With the introduction of solid state devices and digital electronics, ABS technology rapidly progressed and by 1978, Mercedes-Benz introduced the first automobile ABS with electronic wheel speed sensors and hydraulic control valves.
Modern ABSs normally include one to four wheel speed sensors and sensor rings, a hydraulic control unit, an antilock brake controller, a brake proportioning valve, and an ABS warning lamp.
The antilock brake controller collects wheel speed sensor data and, using a proprietary control algorithm in conjunction with the sensor data, determines when wheel lockup is imminent. The measured amount of wheel deceleration is known as slip. A wheel that spins freely has zero slip and a locked wheel has 100% slip. Automotive engineers have determined through testing that 10-20% slip is most effective in maximizing braking force and reducing stopping distance.
When a wheel's speed begins spinning slower than vehicle speed, the ABS enters what is known as “hold” mode. In hold mode, the system prevents any increase in hydraulic pressure at the affected wheel and the line pressure is held at its current level. If the wheel continues to decelerate too quickly while the system is in hold mode, the ABS controller then begins reducing hydraulic line pressure until zero slip occurs. When zero slip is reached, the pressure decrease stops and the existing pressure level maintained. This is known as “release” mode.
While in release mode, the ABS begins to increase pressure to the wheel until slip occurs. Once the desired amount of slip is reached, pressure increases cease and the existing pressure level is maintained. This is known as “apply” mode. If the amount of slip exceeds the desired level while the system is in apply mode, the system returns to hold mode and the cycle repeats.
These frequent pressure decreases and increases that occur to maintain the desired slip during an emergency stop are responsible for the brake pedal pulsations normally associated with modern ABS.
All types of antilock braking systems operate like a conventional hydraulic braking system during normal operation. However, during severe braking, the manner in which the ABS controls line pressure and corrects for excessive slip varies from system to system. ABS can be integrated or independent, two-wheel or four-wheel, and one, two, three, or four channel.
Two wheel systems typically provide antilock capabilities only to the rear wheels and are most often found on pickup trucks and sport utility vehicles manufactured in the early 1990's. Because a large percentage of a vehicle's weight is transferred to the front wheels during heavy braking, the rear wheels are more likely to lock up under heavy braking.
Two wheel systems can be either one channel or two, depending on the number of speed sensors present. In one channel systems, both rear brakes are controlled by the ABS at the same time to control slip. Rear wheel speed is determined by a single speed sensor normally located above the rear differential ring gear, transfer case, or transmission.
Two channel systems utilize two speed sensors, one at each wheel, to determine and control slip of each rear wheel independently. This improves system performance when compared to a one channel system but because the hydraulic pressure at each of the front wheels is not controlled by the ABS, loss of vehicle control can occur if the steering wheels reach 100% slip.
As the name implies, three channel systems utilize three sensors, one at each of the front wheels and a third for both of the rear wheels. The hydraulic pressure at each of the front wheels is independently controlled by two separate hydraulic circuits, while a third hydraulic circuit controls both of the rear wheels.
True four wheel antilock brake systems utilize four individual hydraulic circuits and four speed sensors to control the slip of each wheel independent of the other wheels. This “full” system ensures each wheel receives the exact hydraulic pressure required to maintain maximum braking force while maintaining the desired level of slip and is the most effective ABS currently available.
An independent system uses a conventional hydraulic master cylinder and brake booster along with a separate ABS hydraulic control unit in the brake lines between the master cylinder and wheels. An integrated system replaces the traditional master cylinder with a single hydraulic unit that provides both conventional braking as well as ABS.
All of the systems described above are hydraulic systems, using various wheel sensors as input to a master controller that modulates hydraulic pressure to control braking.
Integrated hydraulic systems such as those described above work well because they are designed as complete systems. Expanding existing ABSs to include other systems, such as trailer braking systems, has been ineffective because of the vast variety of different systems and manufacturers.
Embodiments of the invention address these and other limitations in the prior art.